JPH0449532A - Focus detection method in optical pickup - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus detection method in an optical pickup.

[Prior Art] A focus detection method for an optical pickup is a method of generating a focus error signal for focus control to correctly focus light irradiated onto an optical information recording medium such as an optical disk onto an optical information recording surface. , various methods are known.

Among such focus detection methods, there is a method called the Foucault method.

Generally, it is known that the focus error signal obtained by the focus detection method forms an S-shaped curve when the horizontal axis represents the amount of deviation from the in-focus state, that is, the amount of defocus, and the vertical axis represents the magnitude of the focus error signal. There is. In the central portion of this S-shaped curve, the "defocus amount" and the "magnitude of the focus error signal" have a linear proportional relationship, and the area where this proportional relationship holds is called a linear range.

When the linear range is large, focusing control is possible even if the deviation between the condensing position of the light beam irradiating the optical information recording surface and the optical information recording surface becomes considerably large.

[Problem to be solved by the invention] The Foucault method, which has been known as a focusing control method, has a narrow linear range, so if the amount of defocus becomes a little large, the focusing control will not be performed properly, and the focusing control will become impossible. The problem was that it was unstable.

The present invention was made in view of the above-mentioned circumstances, and
The purpose of this study is to improve the Foucault method and provide a new focus detection method with a large linear range.

[Means to solve the problem] The present invention will be explained below.

The present invention is a focus detection method that is an improvement on the Foucault method as described above, and has the following features.

That is, "the linearly polarized light beam reflected from the optical information recording surface" is made incident on the "Foucault prism made of a birefringent material" and the condenser lens.

The order of incidence may be either the Foucault prism or the condensing lens first.

This luminous flux is then divided into two parts "by the prism action of the Foucault prism." Further, the action of the condenser lens provides convergence to the incident light beam.

Furthermore, each divided portion is separated into an ordinary beam and an extraordinary beam by the birefringence effect of the Foucault prism.

Each divided portion of the luminous flux divided into two is called a luminous flux pair.

This is because each divided portion is composed of an ordinary ray beam and an extraordinary ray beam.

At this time, in each pair of light beams, the ordinary and extraordinary light beams are separated in a direction perpendicular to the prism edge of the Foucault prism.

As a result of the prismatic action of the Foucault prism, each pair of light beams is also separated in a direction perpendicular to the prism edge.

In the end, four convergent beams separated in a direction perpendicular to the prism edge are obtained due to the effects of the Foucault prism and the condensing lens.

The pairs of light beams separated from each other are separately made incident on "a pair of light-receiving elements whose light-receiving surface is divided into two". that time"
Each light-receiving element is arranged so that the beam spots of the ordinary ray beam and the extraordinary ray beam of each beam pair overlap each other, and when the beam irradiated onto the optical information recording surface is focused on the optical information recording surface. The incident state of each pair of light beams is determined so that the light-receiving surface dividing line divides the center of the ordinary and extraordinary light beams into two equal parts.

A focus error signal is generated based on the output of each divided light-receiving surface portion.

[Function] As described above, in the present invention, the light flux pair incident on each light receiving element is separated into a converging ordinary light flux and an extraordinary light flux,
These light beams enter so as to straddle the light-receiving surface dividing line and such that their light beam spots partially overlap each other. Therefore, the light-receiving spot of each light-receiving element has a shape in which the respective light flux spots of the ordinary and extraordinary ray fluxes are connected, and becomes elongated in the direction orthogonal to the light-receiving surface dividing line.

In addition, the ordinary and extraordinary rays are separated in the direction perpendicular to the prism edge of the Foucault prism, so if the light beam irradiating the optical information recording surface deviates from the focused state, the above spot on the light receiving surface will receive light. Displaced in the direction perpendicular to the surface dividing line.

[Example] Hereinafter, description will be given based on specific examples.

FIG. 1(a) shows an example of an optical pickup to which the invention of claim 1 is applied.

A diverging radiation beam from a semiconductor laser, indicated by reference numeral 1 in the figure, is converted into a parallel beam by a coupling lens 2, reflected to the left side of the figure by a beam splitter 3, reflected by a deflection prism 5, and then reflected by an objective lens 6. The light is focused toward the optical information recording surface of the optical disc 7, which is an optical information recording medium. In this way, a light spot is imaged on the optical information recording surface of the optical disc 7 to record, reproduce, or erase information.

The reflected light from the optical information recording surface enters the objective lens 6, is reflected by the deflection prism 5, is transmitted through the beam splitter 3, is rotated by 45 degrees in the polarization plane by the 1/2 wavelength plate 9, and is transmitted to the condenser lens 10. The beam is made into a focused light beam.

This convergent light beam enters the Foucault prism 15.

The Foucault prism 15 is made of a birefringent material such as quartz, and exerts a prism effect and a birefringence effect on the incident light beam.

That is, the incident light beam is divided into two parts, ie, a pair of light beams 171 and 172, in a direction (up and down in the figure) perpendicular to the prism ridge 150 (facing in the direction perpendicular to the figure) by the prism action of the Foucault prism 15. be done.

Furthermore, since the incident light beam is in a linearly polarized state and the polarization direction is oriented at 45 degrees, it is separated into an ordinary light beam and an extraordinary light beam by the birefringence effect of the Foucault prism 15, but the direction of separation is perpendicular to the prism ridge 150. It is the direction. In the figure, the solid line indicates the ordinary ray flux, and the broken line indicates the extraordinary ray flux.

In the end, the light flux emitted from the Foucault prism 15 is four light fluxes, two ordinary light fluxes and two extraordinary light fluxes, and these are all convergent, and the ordinary light flux and the extraordinary light flux constitute a light flux pair. be.

The light beam pair 171 and 172 enters the light receiving element 160.

The light receiving element 160 is composed of two two-part light receiving elements 161 and 162.
It is an integrated system. That is, Fig. 1(b)(c)
As shown in FIG.
The light-receiving surface is divided into parts A and H, and the light-receiving element 1
62 divides the light receiving surface into light receiving surface portions C and D by the light receiving surface dividing line F.
It is divided into. Light-receiving surface dividing lines E and F are prism edge 1
Corresponds parallel to 50.

In FIGS. 1B and 1C, symbols 17B and 17G indicate the luminous flux spots of the ordinary ray flux, and numerals 17A and 17D indicate the luminous flux spots of the extraordinary ray flux.

In FIG. 1(d), reference numeral 40B indicates the intensity distribution of the light flux spot of the ordinary ray, and reference numeral 40A indicates the intensity distribution of the light flux spot of the extraordinary ray. The ordinary ray flux and the extraordinary ray flux are incident on the light-receiving surface such that their luminous flux spots partially overlap each other, as shown in FIG. 1(d). Note that FIG. 1(d) shows a state in which the light beam irradiating the optical information recording surface is focused on the optical information recording surface, and reference numeral 41 indicates a straight line passing through the light receiving surface dividing line and orthogonal to the light receiving surface.

Referring to FIG. 2, (b) of this figure shows the light receiving state of the light receiving element 161 in the focused state.

The ordinary ray flux and the extraordinary ray flux constituting the luminous flux pair 171 are incident on the light receiving surface with the luminous flux spots 17A and 17B partially overlapping each other, but in the focused state, the center of these luminous fluxes (indicated by a black circle) The straight line connecting them is 2 due to the light receiving surface dividing line E.
The incident state is determined so that it is divided into equal parts. Therefore, in this state, the output a from the divided light-receiving surface portion A and the output from the divided light-receiving surface portion B are equal to each other.

FIG. 2(a) shows the light receiving state of the light receiving element 161 when the optical information recording surface is closer to the objective lens 6 than in the focused state. At this time, each luminous flux spot 17A of the ordinary and extraordinary ray flux
, 17B is a divided light-receiving surface portion A while increasing the spot diameter.
, the outputs a and b from each divided light-receiving surface portion A and B
The magnitude relationship is a>b.

FIG. 2(C) shows the light receiving state of the light receiving element 161 when the optical information recording surface is further away from the objective lens 6 than in the focused state. At this time, each luminous flux spot 17A of the ordinary and extraordinary ray flux
, 17B shifts toward the divided light-receiving surface portion B while increasing the spot diameter, and outputs a from each of the divided light-receiving surface portions A and B,
The magnitude relationship of b is a<b.

The light receiving element 162 on which the light flux pair 172 enters is also similar to the above, but the light flux pair 17 is adjusted according to the deviation from the focused state.
The deflection directions of 1,172 are opposite to each other. Therefore, the magnitude relationship of the outputs C9d from the light-receiving surface portions C and D of the light-receiving element 162 is c=d in the focused state, c<d when the optical information recording surface approaches the objective lens side, and cod when the optical information recording surface approaches the objective lens side. Become.

Therefore, each divided light-receiving surface portion A of the light-receiving element 161, 162,
Using outputs a, b, c, and d from B, C, and D, (a-b) + (d-c) E (a + d) - (b + c
), and set the objective lens 6 so that this signal becomes O.
Focusing control can be performed by performing servo control.

Here, it will be explained more specifically.

FIG. 3 is a diagram for explaining the action of the Foucault prism 15.

Assume that the Foucault prism 15 is made of synthetic crystal, the wavelength of the light beam is 780 nm, the apex angle of the Foucault prism 15 is θ, and the refractive index of the Foucault prism 15 for the above wavelength is n with respect to ordinary rays. =1.5378. For the extraordinary ray, n is set to 4.5477.

At this time, the refraction angle θ shown in the figure with respect to the ordinary ray 31 and the extraordinary ray 32. , θ. Using nO°sinθ=sinθ. +”a'Slnθ=s i
nθ.

holds true.

025 degrees and the optical distance 1 from the entrance surface of the Foucault prism 15 to the light receiving surface of the light receiving element 12 is 20 mm, then the distance d between the centers of the luminous flux spots on the light receiving surface.

is do=1·(tanθ, −tanθ.)=16 μm.

On the other hand, the diameter φ of each luminous flux spot on the light receiving surface is φ=2·1.
It is given by 22・λ/NA. 2 in this equation is a coefficient representing the influence of the incident light beam being divided into 1/2 by the Foucault prism 15, λ is the wavelength, and NA is the numerical aperture of the condenser lens 10.

The focal length of the condenser lens 10 is 40 mm, and the beam diameter is 4 mm.
.

If λ=780 nm, φ=38 μm.

That is, in the focused state, a light beam spot with a spot diameter of 38 μm is separated from the center of the light receiving element 161 by a distance of 16 μm.
．． The image will be formed on the light receiving surface of 162.

The relationship between the focus error signal and the defocus amount calculated under this condition is as shown by a curve 40 in FIG. 4.

The linear range is ±6 μm.

In the case of the conventional Foucault method using a Foucault prism that does not have birefringence, the relationship between the focus error signal and the amount of defocus is as shown by the curve 50 in FIG. 4. Linear range is ±
It is 3 μm.

This shows that in this embodiment, the linear range can be doubled compared to the conventional method.

To increase the linear range, the beam spot interval d between the ordinary ray beam and the extraordinary ray beam. The distance d may be increased to the extent shown in FIG. 5, for example, under the condition that the respective light beams overlap.

From a practical point of view, the above distance d. It is preferable that the ordinary ray flux and the extraordinary ray flux overlap at 1/2 of the intensity path. FIG. 1(d) shows this case.

In addition, in the embodiment shown in FIG. 1, if the beam splitter 3 is a polarizing beam splitter and a quarter wavelength plate is provided on the surface of the polarizing beam splitter on the polarizing prism side, the light utilization efficiency can be increased. I can do it.

[Effects of the Invention] As described above, according to the present invention, a novel focus detection method in an optical pickup can be provided. Since the present invention is configured as described above, the linear range is effectively increased compared to the conventional Foucault method, thereby enabling stable focusing control. In addition, in the conventional Foucault method, it was not possible to set the linear range as a design value, but in the present invention, by adjusting the distance between the Foucault prism and the light-receiving element, the distance between the centers of the ordinary and extraordinary ray fluxes on the light-receiving surface (the third Since the linear range can be adjusted by adjusting d.) in the figure, it is possible to set the linear range by design.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining an example of an optical pickup to which the present invention is applied, FIG. 2 is a diagram for explaining focus detection in the optical pickup of FIG. 1, and FIG. 3 is a diagram for explaining a Foucault prism according to the present invention. FIG. 4 is a diagram for explaining the invention in detail in relation to the optical pickup shown in FIG. 1. FIG. 5 is a diagram for explaining the overlapping of ordinary and extraordinary rays This is a diagram.

Claims (1)

[Claims] In an optical pickup, a linearly polarized light beam reflected from an optical information recording surface is incident on a Foucault prism made of a birefringent material and a condensing lens to be divided into two, and each division is divided into two. By separating the part into a convergent ordinary beam and an extraordinary beam, each beam is composed of an ordinary beam and an extraordinary beam, and the ordinary beam and the extraordinary beam are orthogonal to the prism ridge of the Foucault prism. Obtain two pairs of light beams that are separated in the direction, and make each of the above pairs of light beams incident one by one on each of the pair of light-receiving elements whose light-receiving surface is divided into two. When the light beam spots of the light beams partially overlap each other and the light beam irradiated onto the optical information recording surface is focused on the optical information recording surface, the light receiving surface dividing line of each light receiving element is different from the ordinary light beam. A focus detection method characterized in that the incident conditions for each pair of light beams are determined so as to divide the center of the light beam into two, and a focus error signal is generated based on the output of each divided light receiving surface portion.